3.45 pm
An integrality theorem of Grosshans over arbitrary base ring
Wilberd van der Kallen
Utrecht University.
07-08-13

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Abstract

We revisit a theorem of Grosshans and show that it holds over arbitrary commutative base ring $k$. One considers a split reductive group scheme $G$ acting on a $k$-algebra $A$ and leaving invariant a subalgebra $R$. If $R^U=A^U$ then the conclusion is that $A$ is integral over $R$.